Solid-state image sensors are widely used in a number of optoelectronic applications including digital cameras, cellular phones, and optical navigation or pointing systems, such as optical computer mice, trackballs, touch screens scroll bars and the like. Conventional solid-state image sensors typically include an array of photosensitive elements, such as charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) photodiodes. Typically, one or more adjacent photosensitive elements are coupled in an elementary unit or picture element commonly known as a pixel.
A conventional image sensor will now be described with reference to FIGS. 1A and 1B. Referring to FIG. 1A, a portion of the image sensor 100 is shown in cross section. The image sensor 100 typically includes a semiconductor substrate 102 in or on which an array of pixels 104A, 104B has been formed. Each pixel 104A, 104B shown in these figures can include one or more adjacent photosensitive elements, for example, CCDs or photodiodes (not shown). The pixels 104A, 104B convert incoming light 106 from an imaging plane or a light source 108 into electrical signals via the CCDs or photodiodes. The substrate 100 is covered by one or more dielectric layers 110, which may be substantially transparent to the incoming light 106, or may include a number of transparent portions 112, as shown, extending therethrough to transmit the incoming light to the pixels 102A, and 102B.
Incident light 106 striking the top surface of the sensor 100 transmitted through the underlying dielectric layer 110 or the transparent portion 112 of the dielectric down to the underlying pixel 102A or 102B. However, it is a common occurrence for the incident light 106 to strike the top surface of the sensor 100 surface at a variety of angles. Thus, light 106 striking the image sensor 100 at a perpendicular or nearly perpendicular angle to a surface thereof is propagated unimpeded to the underlying pixel 102A. However, where the dielectric layer 110 is substantially transparent to at least some wavelengths of the incoming light 106, and part of the light 106A strikes the image sensor 100 at other than a perpendicular angle, a portion of the light can be transmitted to the adjacent pixel 102B rather than the pixel 102A underlying the strike location. This undesirable effect is commonly called cross-talk and results in reduced accuracy or image resolution of the image sensor 100. Moreover, even where this scattering of light in dielectric does not result in striking an adjacent pixel 102B causing cross-talk, the loss or non-sensing of the light by the pixel 102A underlying the strike location results in reduced photo-efficiency or sensitivity of the image sensor 100.
Referring to FIG. 1A, one approach used in conventional image sensors 100 to minimize if not eliminate cross-talk is the introduction of one or more intermetal dielectric (IMD) layers 114A, 114B overlying or between the dielectric layers 110 to reduce or impede the transmission of light therethrough. FIG. 1B, illustrates a top planar view of a portion of the image sensor 100 of FIG. 1A, showing a top IMD layer 114A and lower IMD layer 114B in cut-away. This solution is not wholly satisfactory in that depending on the number, location and size of the IMD layers 114A, 114B not all of the light 106A striking at a highly oblique angle will be stopped, and the IMD layers do not improve the photo-efficiency or sensitivity of the image sensor 100. Moreover, the use of multiple metal IMD layers 114A, 114B requires they be isolated from one another and conducting elements of the image sensor. This requirement in turn causes an increase in the pixel size, or a decrease of the number of pixels within the image sensor.
FIGS. 2A and 2B illustrate another approach used in conventional image sensors 200 to reduce cross-talk and improve photo-efficiency or sensitivity of the image sensor. In this approach the image sensor 200 further includes an air-gap 202 or ring of dielectric material (not shown), in a transparent dielectric layer 204 surrounding each pixel 206A, 206B on the substrate 208. Referring to FIG. 2A, light 210 from a light source 212 striking the image sensor 200 at a perpendicular or nearly perpendicular angle to a surface of the image sensor 200 is propagated unimpeded to the underlying pixel 206A. Due to differences in refraction between the material of the dielectric layer 204 and the air-gap 202, light 210 striking at a less than perpendicular angle is reflected from an interface between the dielectric layer 204 and the air-gap 202, and back towards the underlying pixel 206A, thereby reducing cross-talk and improving photo-efficiency of the image sensor 200. However, where the incoming light 210 strikes the image sensor 200 at a highly oblique angle, the difference in refraction is insufficient to reflect all of the light, and a portion of the light 210A can be transmitted to the adjacent pixel 206B rather than the underlying pixel 206A.
Yet another common approach in conventional image sensors to reduce cross-talk and improve photo-efficiency uses a number or an array of micro-lens (not shown) to focus light onto each element or pixel of the image sensor. This approach is also not wholly satisfactory for a number of reasons. First, as with the other approaches outlined above for light striking the micro-lens array at a highly oblique angle, the lens may be insufficient to redirect all of the light toward the underlying pixel, and away from the adjacent pixel. Moreover, the fabrication of the micro-lens array and/or mating the array with the pixels of the image sensor can significantly increase the cost and/or decrease the throughput and yield of working devices.
Accordingly, there is a need for an image sensor and method of fabricating the same that substantially eliminates cross-talk between adjacent pixels while increasing or improving photo-efficiency or sensitivity of the image sensor. It is further desired that the sensor does not include shields that increase pixel size or reduce the number of pixels of the image sensor. It is still further desirable that the sensor does not require micro-lenses, which increase cost of fabricating the image sensor.
The present invention provides a solution to these and other problems, and offers further advantages over conventional image sensor and method of fabricating the same.